Scientific and public interest in the biology and ecology of gelatinous zooplankton (jellyfish) has increased significantly over the past two decades. This Theme Section represents the proceedings of the 4th International Jellyfish Blooms Symposium held in Hiroshima, Japan, in June 2013; to date, this was the largest meeting ever of the international jellyfish community. Research highlights presented in this volume include studies on jellyfish blooms at various spatiotemporal scales, jellyfish population dynamics, physiology and ecology, interactions between jellyfish and predators, jellyfish age, growth and diversity, and the fate of jellyfish biomass.